#include <iostream>

using namespace std;

int main(int argc, const char* argv[])
{
	int N;
	cin >> N;
	int a, b;
	int gap = 0;
	for (int i = 0; i < N; i++) {
		cin >> a >> b;
		if (i == 0) gap = b - a;
		if (gap <= 0 || gap != b - a) {
			cout << -1 << endl;
			return 0;
		}
	}
	cout << gap << endl;
	return 0;
}